1. Field of the Invention
The present invention relates to a stretched film of a thermoplastic resin and a process for producing it, wherein a charge potential is controlled during production of the film, and electrostatic disturbance is less in the secondary processing such as printing, coating, vapor deposition, sticking, cutting, etc.
2. Description of the Background
In order to improve the wetting property of the film surface and enhance the adhesion of inks, coating agents, adhesives, etc., it is known to conduct discharge processing on the film surface by applying a high voltage of high frequency to the discharge electrode prior to the secondary processing for the stretched film of thermoplastic resin. In practice, this process has widely been employed in production of film and in the secondary processing. In such a case, the stretched film of thermoplastic resin is considerably charged while passing through the discharged electrodes and the rolls (earth electrodes) since the film is generally of an insulating material. Thus, a discharger is disposed after treatment of discharging to remove a static electricity accumulated on the film surface. Alternatively, a transfer-type antistatic agent such as glycerin fatty acid esters is kneaded into the thermoplastic resin so as to bleed out on the film surface to remove the static electricity.
Resin-based stretched films containing a large number of fine voids therein, which can be prepared by adding an inorganic or organic fine powder to a thermoplastic resin and stretching the resulting resin at a temperature lower than the melting point of the resin, have widely been used for many reasons, for example, high concealment made possible due to high opacity, successful weight reduction, ease of various types of printing depending on its cushion property, etc. In such films, however, the thermoplastic resin generally has a discontinuous structure due to voids in direction of the film thickness (FIG. 1). Thus, it was found that the charge accumulated inside the film by discharge processing could not easily move onto the surface and could not be thoroughly removed with a discharger.
Even if an antistatic agent was kneaded into or coated on the stretched film of void-containing resin, the charge accumulated inside the film by discharge processing could not easily removed, and it remained charged. Many static disturbances have been reported such as spark discharge caused by static electricity on the film surface, difficult arrangement of the cut film, etc., when the secondary processing is made in a low humidity environment (e.g., winter season) where no antistatic agent is used.
An object of the invention is to provide a stretched film of a void-containing resin in which no charge is accumulated and which scarcely produces electrostatic disturbance during the secondary processing in a low humidity environment.